Heat's
on silicon

By
Kimberly Patch,
Technology Research NewsMoore's Law has been under attack of late,
and probably for good reason.

In 1965 Intel co-founder Gordon Moore observed that the number of transistors
in a silicon computer chip was doubling every year. What has become known
as Moore's Law has held true -- with an adjustment to 18 months rather
than every year -- for more than three decades.

A researcher from Texas A&M University has shown that the laws of physics
are close to catching up with Moore's Law in a way not widely thought
about. The culprit is heat, according to Laszlo Kish, an associate professor
of electrical engineering at Texas A&M University.

More conventional predictions of Moore's Law's demise concentrate on the
laws of physics that indicate the difficulty of etching increasingly smaller
transistors into silicon. Today's circuits measure 100 nanometers, or
about 50 times smaller than the diameter of a red blood cell. To keep
pace with Moore's Law, circuit size will have to shrink to about 40 nanometers
by the end of the decade.

According to Kish's calculations, the overlooked effect of heat on will
make circuits of that size unreliable.

As chips get faster and carry out more calculations per second, they must
also dissipate more energy.

"Today the chips are at the limit as far as power dissipation is concerned,"
said Kish.

To compensate for this, chipmakers have been decreasing chip voltage in
order to keep power dissipation manageable, and making the chips more
sensitive so that they work using the lower-voltage signals.

Amps are measures of electrical current, Watts are units of power, and
voltage is the difference in electrical potential between two points in
a circuit. The higher the voltage, the more Watts are produced per amp.

But as circuits continue to decrease in size and their supply voltage
continues to grow weaker, energy generated by thermal noise, or heat,
will start to interfere with electrical signals, which will increase chip
errors, said Kish. "There is a fundamental interrelation between noise,
information, speed and dissipation," he said.

Not only is there a limit to how low chip voltage can go, but that limit
is closer than we think, said Kish.

Decreasing transistor size in the complementary metal oxide semiconductor
(CMOS) chips used in most computers today from 100 nanometers to 40 nanometers
will increase thermal noise by about a factor of three, which is enough
to cause serious problems, said Kish. This means that the thermal noise
is poised to become a serious issue at the beginning of the next decade,
he said. "CMOS technology is close to [its thermal] limit," he said.

The effects could already be causing problems in today's smallest integrated
circuit prototypes, according to Kish.

The significance of the temperature effect on digital circuits came as
a surprise, and only came to light after detailed calculations connected
with projects in nanoelectronics and nanotechnology, said Kish. "I expected
something like that for nanoelectronics, [but the effect on CMOS chips]
was a great surprise," he said. "I have... grown up with the belief that
thermal noise is not an issue in digital circuits."

The thermodynamics problem has historically been overlooked because the
original digital circuits were so big and slow that they had a very large
tolerance for thermal noise. "It was obvious for everybody that thermal
noise was not an issue," said Kish. Over time circuits have shrunk drastically
in order to shorten the distances between circuits, which increases the
number of calculations a chip can make in a second.

There's not a lot that can be done to get around the thermal limitation,
said Kish. Today's silicon technology is the only commercially viable
alternative for reaching the 40-nanometer size range by the end of the
decade, he said.

It is possible to continue to successfully shrink the size of silicon
transistors only if the clock frequency, or speed of the chip is decreased,
said Kish. This could work if new chip architectures and software were
developed that took greater advantage of parallel processing -- using
several chips at once to do the work of one, he said.

The results open up several questions, said Kish. Chip technology has
remained largely the same for the past 30 years. The thermal limit may
push the field toward entirely new architectures. One possibility is low-power
computation. Another avenue is to look to biology, said Kish. "Can we
do like the brain [and use] the noise for carrying the information?" he
said.

There are many research efforts aimed at alternative chip architectures.
There are two issues that may keep these architectures from emerging in
practical devices very soon, however. Many are based on materials that
are more expensive than silicon, the main substance of computer chips
for the past three decades. And most would require completely new manufacturing
processes.

Kish is currently working to understand the noise observation more closely,
and relate it to similar observations on the quantum scale of atoms and
particles. "I'm working on a paper which has a unified approach to quantum
and classical information," he said.

Kish published the research in the December 2, 2002 issue of Physics Letters
A. The research was funded by Texas A&M University.